To simplify the following discussion, the present invention will be discussed in terms of displays utilized in head mounted computer displays; however, it will be apparent to those skilled in the art from the following discussion that the present invention may be applied to other types of displays. Head-mounted computer displays may be viewed as "eye glasses" that are worn by the user to view images created by a computer or other image source. The image seen by each eye is generated on a display screen having a two dimensional array of pixels.
It is advantageous to minimize the distance between the display and the eye of the viewer to minimize the portion of the display that extends from the viewer's face. Large overhanging displays are uncomfortable to wear. In addition, optical systems based on reflectors are preferred since such systems provide high quality optical imaging at a cost that is much less than systems based on lenses.
Prior art systems that combine reflective optics with short display to eye distances are known to the art. These systems typically utilize partially reflecting optical surfaces to fold the optical path so that the distance from the viewer's eye to the display is minimized. Such a system is described in U.S. Pat. No. 5,644,436. For such systems to operate, the direct light emitted by the display must be blocked. Prior art systems typically utilize polarization filters and quarter wave plates to block the direct light from reaching the eye. Unfortunately, quarter wave plates only rotate the polarization vector of the light through precisely 90 degrees for specific wavelengths. Light having wavelengths that differ from the design wavelength is rotated either through slightly more than 90 degrees or slightly less than 90 degrees. Accordingly, these prior art systems do not block all of the light coming directly from the display and the viewer sees "ghost" images.
Broadly, it is the object of the present invention to provide an improved folded optical system based on reflective optical imaging elements.
It is a further object of the present invention to provide an optical system in which light from the display being imaged is blocked from reaching the viewer's eye independent of the wavelength of the light.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.